Mars is often regarded as an arid world covered by deserts. However, like Earth, the Red Planet has two large ice caps at its north and south poles. If people are one day to live on Mars, it is important to know how much frozen water is in these ice sheets. This is just what is being discovered by the MARSIS instrument on ESA’s Mars Express orbiter.

In order to measure the thickness of the southern ice sheet, the radar instrument sends invisible signals toward the surface and records the echoes. In this way, the radar is able to ‘see’ through the layers to the bottom of the ice. More than 300 of these ‘slices’ have been made through the deposits of ice and dust that cover the pole.

Thickness of Mars' South Polar Layered Deposits

We now know that the southern cap is up to 3.7 km thick. If all the ice melted, the entire planet would be covered by an ocean about 11 metres deep. Despite the weight of this massive ice sheet, the rock beneath the ice has not been pushed down – evidence that it is stiffer than Earth’s crust.

Ongoing radar studies of the north polar cap have shown that it is about 1.8 km thick. The thin layers of water ice and dust at both poles seem to be caused by seasonal and long-term changes in the climate. Although the polar caps hold most of the known water on modern Mars, other places on the planet may have been wet enough to support life in the distant past.